Film actresses have exclaimed all manner of things in interviews, but it’s safe to say that until now none has ever revealed the following: “the inside of my butt cheeks were literally rubbed raw!” Thandie Newton is telling the horror story of her Christmas vacation in Mexico, when she made the mistake of wearing thong underwear while sharing a saddle with her three-year-old daughter Nico on a trail ride. “I suffered what my older daughter called ‘a lethal wedgie,’” Newton offers by way of elaboration.

Though Newton, 35, has a beauty that is singular, she’s that type of gorgeous, brainy girl who makes scatological jokes because, one, she just thinks they’re funny, and two, innocent vulgarity renders her more accessible. This quality—her willingness to play dirty with the boys—is what allowed Newton, an award-winning dramatic actress, to join fellow Brit Simon Pegg in his latest comedic outing, Run, Fatboy, Run. In the romantic comedy, directed by David Schwimmer, Newton plays Libby, a woman left at the altar by Pegg’s schlubby Dennis, who years later attempts to woo her back from a suave new suitor. “It was very different for me,” she says, no doubt thinking back on the meaty, serious roles she’s had in films like Beloved, Besieged and Crash, for which she won a British Academy of Film and Television Arts award in 2006.

It’s a wonder Fatboy got made at all. Newton describes the shoot as an endless exchange of practical jokes between Pegg and herself, involving Marmite, underwear, sewn-up shirt cuffs and, ultimately, a classic vodka-disguised-as-Evian maneuver orchestrated by Newton at the film’s London press junket. Reflecting on all the schoolyard shenanigans, Newton paraphrases an old George Bernard Shaw line favored by her husband, the writer and director Oliver Parker: “We don’t stop playing because we get old; we get old because we stop playing.”

If play is her secret to youth, Newton has her two daughters to thank, even though the oldest, seven-year-old Ripley, is a “tremendously sophisticated person,” Newton says. “She’s always saying to me ‘Mummy!’ when I swear.” Ripley was named after Sigourney Weaver’s character in Alien, an unconventional homage that has caused Newton and her husband some unexpected stress. “A major shame flash moment” is how Newton describes her encounter with Weaver. “We were introduced by a friend, and when we went to her apartment we, who named our child after her, felt like we should be genuflecting. She was so cool, and incredibly nice, but it was just too much.”

Newton has handled brushes with other celebrities with more aplomb. She recently completed filming RocknRolla, directed by Guy Ritchie. (“He’s unflappable,” she says.) Ritchie’s enthusiasm for jujitsu rubbed off on little Ripley, who practices the martial art at the same London center as Ritchie’s son, Rocco. The result? Newton and Ritchie’s wife, Madonna, are “jujitsu mums” together, cheering their offspring from the sidelines. “What a cool way to meet Madonna,” Newton marvels.

Consumed though she is with family and work, Newton has been increasingly active in environmental and nonprofit causes. She recently visited Mali with the charity World Vision to raise awareness about water sanitation, citing as motivation her own connection to Africa through her Zambian mother.

And yet Newton is also a savvy Londoner, and has interests to match. She confesses to having splurged on a large work by Cornish painter Anthony Frost (“He’s proper!” she jokingly boasts), and is a devotee of several British fashion designers, especially Nathan Jenden. “His designs are clever and wearable,” she enthuses.

In fact, Newton often wears Jenden’s frocks while treading the red carpet. Accomplished star though she is, Newton frets that the awards season might be occasion for another “shame flash moment” with some other celebrity. Surely not? With a self-deprecating giggle, the Cambridge-educated Newton repeats that, like the rest of us, she is highly subject to gaffes: “I promise you, it’s nothing that a little Champagne can’t organize!”